Block copolymer rubber based hot melt pressure sensitive adhesives (PSA's) are well known as showing unique viscosity-temperature profiles at hot melt coating and end use temperatures. Rubber based hot melt pressure sensitive adhesives are typically tackified styrene-isoprene-styrene, styrene-butadiene-styrene, styrene-butadiene, and/or styrene-isoprene block copolymers. In such block copolymers, the styrene end blocks form physical cross-link sites or domains which can be temporarily eliminated by heating thereby allowing the rubber to flow, and to reform upon cooling for end use requirements. This reversible cross-linking contributes to the unique processability characteristics of rubber based pressure sensitive adhesives.
Rubber based pressure sensitive adhesives have poor long term aging properties. By contrast, known acrylic based pressure sensitive adhesives exhibit excellent aging properties. However, they do not share the unique viscosity-temperature behavior of rubber based hot melt PSA's and they are more difficult to process.
Attempts have been made to develop hot melt acrylic pressure-sensitive adhesives with properties similar to rubber based adhesives. One approach has been to make high molecular weight polymers which exhibit good cohesive strength at ambient temperature. These high molecular weight polymers, however, create problems in the hot melt coating process due to high viscosity. One must therefore resort to processing using solvents to achieve acceptable coating viscosities. Solvents defeat the benefits of hot melt adhesives and create problems of pollution, waste generation and disposal.
Efforts to produce a processable low viscosity acrylic hot melt PSA have generally followed two paths.
One path has involved synthesizing polymers that require post coating irradiation to build up a cross-link network and the desired adhesive properties. This method requires extra processing time and equipment.
The other path involved synthesizing polymers that could be subsequently cross-linked by the addition of metal cation. Several techniques have been disclosed in the prior art which involve improving the cohesive strength of acrylic hot melt adhesives by making pressure-sensitive adhesive ionomers and incorporating metal cations such as zinc or cobalt. Such compositions, however, usually show high melt viscosities which prohibit use as hot melt adhesives.
A cross-linking mechanism between metal ions and carboxylate groups has been described, for instance, in U.S. Pat. No. 4,423,180, incorporated herein by reference, where melt viscosity has been lowered by adding a third component, namely an o-methoxy substituted acid or its mineral salt. This chelating mechanism was first disclosed in U.S. Pat. No. 3,331,729, incorporated herein by reference, where a zinc resinate was used to chemically coordinate with carboxylic groups of the polymers.
Other approaches were described in U.S. Pat. No. 3,740,366 for a pressure sensitive adhesive ionically cross-linked with polyvalent metals, such as zinc salts, and U.S. Pat. No. 3,769,254, for having improved cohesive strength PSA's by combining chelating metal alkoxide with carboxylic containing polymers. Similar approaches have been described in U.S. Pat. No. 3,925,282. Each of said patents is incorporated herein by reference.
U.S. Pat. No. 4,360,638 disclosed a similar mechanism of polymer and metal salt interaction by adding an o-methoxy substituted aryl acid to control the viscosity and cross-linking sites. Similar mechanisms have been described in U.S. Pat. No. 4,423,182 and U.S. Pat. No. 4,851,278 each of said patents also incorporated herein by reference.
There is a need for acrylic pressure sensitive adhesives which exhibit viscosity-temperature profiles which make them useful in hot melt adhesive coaters typically employed for rubber based hot melt adhesives.